Reactor redesign faulted in San Onofre report

A redesign of the troubled replacement steam generators at the San Onofre nuclear plant added 377 more heat transfer tubes than the original generators, according to an analysis commissioned by an environmental group.

The report on San Onofre by Fairewinds Associates, a Vermont-based consultant that has worked with groups critical of nuclear power, suggests that "imprudent design and fabrication decisions" may be to blame for accelerated wear on generator steam tubes. Friends of the Earth commissioned the analysis.

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Plant operator Southern California Edison is still trying to pinpoint the cause of tube degradation at the dual-reactor seaside plant near the border of San Diego and Orange counties.

Generator tubes within both reactors have shown signs of wear from rubbing against each other and also against support structures.

Four 1980s-era generators at San Onofre -- two in each reactor -- were replaced in 2009-2011 with new units manufactured by Mitsubishi Heavy Industries of Japan.

The analysis found that the number of tubes in each was increased from 9,350 to 9,727, requiring additional holes in a tubesheet at the generator's base. Making room for the additional tubes also required the removal of supporting cylinder designed to prevent vibrations. The changes altered the generator's pressure and flow, the report said.

"Fairewinds believes it was this management decision to increase the number of tubes that lead in turn to a series of cascading design changes that created the serious problems San Onofre is experiencing in 2012," the analysis stated.

Edison declined to comment on the content of the report, releasing the following statement: "Southern California Edison continues to prioritize the health and safety of the public and our workers during this very thorough inspection, analysis and testing process of both units at San Onofre."

The Nuclear Regulatory Commission has said it was made aware in 2006 of the extent of proposed design changes to the replacement generators and that the agency was satisfied the equipment would meet federal requirements for safe operation.